Testing for substance properties is integral to any science. This step was traditionally destructive in nature. Examples include pulling a metal apart until fracture to determine its strength, combusting a chemical to determine its elemental composition and digesting food to determine its toxicity. Such methods are not practical when considering substances that are expensive, limited in availability, substantially large or small, statistically variable and those which can additionally yield undesirable by-products resulting from destructive testing. To that effect, non-destructive testing (NDT) is a widely favored method. One of the most powerful NDT techniques is Raman spectroscopy.
Raman spectroscopy is a mature scientific method that can offer characterization of any substance in any physical state in addition to real-time reaction process monitoring. Raman spectroscopy relies on molecular vibrations that uniquely scatter the incident electromagnetic radiation. Since vibration is highly dependent on boundary constraints, it is possible to characterize crystal lattice structures and compositions of matter quite effectively. However, the scattering intensity due to these molecular vibrations may be too weak to detect even with sophisticated Raman equipment. As a result, a constantly increasing effort is being made to enhance the Raman scattering signal.
To enhance the Raman scattering signal, it was found that the materials' electronic structure holds most of the potential. That is, the collective oscillation or resonance of conductive band electrons can stimulate an electromagnetic enhancement. It was found that gold (Au) and silver (Ag) possess such desirable electronic structures. To localize this resonance condition, it was necessary to confine the surface plasmons to features that are smaller than the wavelength of incident light (i.e. to nanostructures). As a result, enhancement factors on the order of 104 have now become available with Au and Ag nanoparticles (NPs). The field of study to enhance the Raman spectra is now commonly referred to as Surface Enhanced Raman Spectroscopy (SERS).
The current state of the art in Raman spectroscopy, as has been for decades, is to use single Au and Ag nanoparticles to enhance the acquired spectrum. Well-developed two-dimensional (2-D) nanomanufacturing techniques have shown Raman enhancement but with drawbacks. For example, recently published chemical methods to control NP aggregation demand precise solution control while commonly requiring additional stabilizers to regulate surface features. Stabilization is even more critical for bigger Au NPs. Modifying the solution and functionalizing the NPs in this way may also interfere and mislead the acquired Raman spectrum. Consequently, smaller Au NPs are used despite worse SERS performance. Alternatively, using Ag can improve SERS performance but the inherent oxide layer causes severe response fluctuations. Moreover, the highest plasmonic activity of Ag is at around the 532 nm excitation wavelength. This regime is highly susceptible to sample fluorescence and Raman signal deterioration.
Moreover, health and environmental impacts of nanotechnology, to date, have not been evaluated. This is potentially a serious problem for the future of SERS and Raman spectroscopy in general should it become apparent that Au and Ag NP containing systems are detrimental to human well-being in which case new SERS materials would need to be developed. As an alternative, titania is thermodynamically stable, attracts water and water soluble molecules, it is favorable for biomolecular bonding and it is corrosion resistant with a stable oxide surface. These characteristics are also favorable for SERS since the system has to remain stable under laser excitation. It is also desired for the targeted molecules to be in close proximity to the regions of surface enhanced electromagnetism. In general, titania is already a widely commercially available material, being used in cosmetics, pigments, water treatment, solar energy conversion and ultra-violet ray blocking. Manufacturing benefits such as cost, sustainability, high production and efficiency may be realized. TiO nanowires are already used in some medical devices to enhance surface cellular functions as disclosed by U.S. Pat. Publication No. 20050221072 and U.S. Pat. Publication No. 20050038498.